Cellular communications systems are commonly employed to provide voice and data communications to a plurality of mobile units or subscribers. Analog cellular systems, such as designated AMPS, TACS, ETACS, NMT-450, NMT-900, CP1, CP2 and Telepoint have been deployed successfully throughout the world. More recently, digital cellular systems, such as designated IS-54B in North America and the pan-European GSM system as well as ADC, JDC, DE and DCT cellular systems, have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
A cellular radio communications system generally includes one or more stations or units, typically referred to as cellular terminals, one or more base stations and a mobile telephone switching office. A typical cellular radio communications system may include hundreds of base stations, thousands of cellular terminals and more than one mobile telephone switching office. The mobile telephone switching office and the plurality of base stations associated therewith generally define the cellular radio network.
The cellular radio network is typically divided into cells which include one base station and the cellular terminals with which the base station is in communication. Each cell will have allocated to it one or more voice channels and, if the cellular radio network is digital, one or more dedicated control channels. For example, a typical cell of a digital cellular radio network has one control channel and 21 voice/data or traffic, channels. The control channel is a dedicated channel for transmitting cell identification and paging information while the voice channels carry the voice and the data information, such as, for example, via a Dual Tone Multi Frequency tone signalling scheme. Note that "channel" may refer to a specific carrier frequency in an analog system or to a specific carrier/slot combination in a hybrid TDMA/FDMA system, such as IS-45 or in GSM.
The mobile telephone switching office is a central coordinating element of the overall cellular radio network. The mobile telephone switching office typically includes a cellular processor and a cellular switch which provides the interface to an associated public switched telephone network. The mobile telephone switching office also generally includes a Home Location Register. The Home Location Register includes a memory device for storing data relating to the subscribers to the cellular system. This data may include the location of the subscriber as well as any specific services requested by the subscriber, such as call waiting or call hold.
Each base station of a cellular radio network generally includes a control unit and an antenna tower. The control unit comprises the base station electronics including the radio control group, the exchange radio interface, a primary power supply for converting electric power from the AC grid to power the individual components within the base station and a backup power supply.
The exchange radio interface of the control unit of the base station electronics provides signals between the mobile telephone switching office and the base station. For example, the exchange radio interface receives data from the radio channel group and transfers the data to the mobile telephone switching office via a dedicated communications link. In the reverse direction, the exchange radio interface receives data from the mobile telephone switching office into the radio channel group for subsequent transmission to a cellular terminal.
The radio control group includes the electronic equipment necessary to effect radio communications. A radio control group typically includes one transmit/receive module for each voice and control channel as well as one signal strength receiver. Each transmit/receive module includes a transmitter, receiver and control unit. The transmit/receive modules are not typically frequency agile and operate, instead, on only one predetermined channel. Control signals from the exchange radio interface are received by the individual control units of the transmit/receive modules. In addition, voice and data traffic signals from the transmit/receive modules are routed over separate interfaces to the exchange radio interface.
Thereafter, each individual transmitter for control and voice signals is connected to a transmit combiner. The transmit combiner combines all of the input signals onto a signal output coupled through a coaxial cable to the transmit antenna. On the receive side, each of two receive antennas is coupled to a respective receive combiner where the signals received are separated according to frequency and passed on to the individual receivers in each of the transmit/receive modules.
The base station handles the radio communication with the cellular terminals. In this capacity, the base station functions chiefly as a relay station for data and voice signals. The base station also supervises the quality of the communications link and monitors or receives signals from the cellular terminals. A typical base station is Model No. RBS882 manufactured by Ericsson Telecom AB Stockholm, Sweden for the CMS8800 Cellular Mobile Telephone System. A full description of this analog cellular network is provided in Publication No. EN/LZT 101 908 R2B, published by Ericsson Telecom AB.
Through the cellular radio network, a duplex radio communications signal or link may be established between two cellular terminals or, between a cellular terminal and a landline telephone terminal. In particular, a communications link is established by a cellular radio communication system, in part, via a voice channel which is established between a cellular terminal and the cellular radio network. The communications link may be completed by the interconnection of the open voice channel to a second voice channel between the cellular radio network and another cellular terminal serviced by the cellular radio network. Alternatively, the communications link may be completed by interconnection of the open voice channel with a voice channel established between the cellular radio network and the public switched telephone network and, in turn, with a terminal associated with the public switched telephone network, such as a landline telephone terminal.
A feature node may also be associated with the mobile telephone switching office of the cellular radio communications system or with another communications network, such as a public switched telephone network. For example, the feature node may be incorporated within or associated with the Home Location Register of the mobile telephone switching office. The feature node provides predetermined functions to the cellular terminals or to the terminals associated with the public switched telephone network, such as, for example, establishing an efficient communications link between terminals, such as via intermediate base stations, or providing voice-control speech information and number translation services as well as facilitating the establishment of conference telephone calls. Still further, a feature node may provide paging services for the user of a cellular terminal and may facilitate the establishment of alternative communications links if the primary communications link is unavailable. For example, a feature node is described in more detail in U.S. Patent application Ser. No. 018,268 entitled "A Method of Establishing Cooperation With a Functionality" and U.S. Patent application Ser. No. 018,223 entitled "A Method of Organizing Communication", both of which were filed on Feb. 16, 1993 and both of which are incorporated herein by reference.
Typically, the voice channels which establish the communications link between the cellular terminal and the cellular radio network are disconnected upon the termination of the communications. For example, the voice channel between a cellular terminal and a cellular radio network and, in particular, the voice channel between the cellular terminal and the respective base station is disconnected upon the termination of communications with the cellular terminal. More specifically, the user of the cellular terminal may terminate the session by hanging up or turning off the cellular terminal or the cellular radio network and, in turn, the cellular terminal may receive an "end communications" signal from the other terminal indicating that the communications has been discontinued. Thereafter, the disconnected voice channel may be reused by the same or another cellular terminal. As known by those skilled in the art, communications may also be terminated due to low or insufficient signal strength or by the use of the cellular terminal exiting the service area of the cellular radio network.
As explained, a cellular terminal is serviced by and communicates with the base station of the cell in which the cellular terminal is located. Each base station has, however, only a limited number of voice channels with which to communicate with the cellular terminals in its cell. Accordingly, the cellular terminals compete for the limited number of voice channels and, if demand for the voice channels exceeds the number of available channels, a cellular terminal will not be able to establish communications with the cellular radio network. Instead, the cellular terminal will be forced to wait until a channel subsequently becomes available. In addition, even if a voice channel is available, the establishment of the desired communications link consumes time and delays initiation of the communications.